In an OFDM signal receiving system, factors deteriorating an OFDM signal exist. The factors are white noise, phase noise, multipath inter-symbol interference, inter-code interference, rapid moving fading, spurious interference, channel interference, for example. Improvements for eliminating these deteriorating factors have been studied in various technical areas.
The improvements are divided into two types. Some of the improvements are in time domain before Fast Fourier Transform (FFT). Others of the improvements are in frequency domain after FFT. In both types of improvements, reception quality measurement is required to determine the effect of each of the improvements.
Japanese Patent Application Publication No. 2004-336279 and Japanese Patent Application Publication No. 2008-118567 disclose an OFDM signal receiving system which calculates S/N (Signal to Noise ratio) from an OFDM signal in the frequency domain after FFT, and uses the S/N to determine the reception quality of received data.
There is a case in which S/N data is measured by calculating deviation from original reference point for each demodulated data. In this case, a deviation from original reference point set by a transmitting side is calculated. Therefore, generally, as a carrier for measuring S/N data, a subcarrier of low modulation degree such as BPSK (Binary Phase Shift Keying) or non modulation is selected rather than a subcarrier modulated by a modulation method having high modulation degree such as 64QAM (Quadrature Amplitude Modulation).
The S/N data calculated in this way is used for FFT window search. By the FFT window search, an optimal position of FFT window is set for an OFDM signal in the time domain.
The FFT window search will be described with reference to FIGS. 1 to 2.
FIG. 1 illustrates a main wave (direct wave) 200, a delay wave (reflected wave) 201 which hits a reflecting object and reaches a receiving side through a path different from that of the main wave, and the FFT window.
The main wave 200 includes a guard interval 202 and a valid symbol period 204. The delay wave 201 includes a guard interval 203 and a valid symbol period 205. The delay wave 201 at this time indicates a wave having a delay time ti in the guard interval 202 (=tg) of the main wave.
By moving the FFT window in the time axis direction and performing the FFT window search, a demodulation timing (FFT window position), at which S/N of an FFT output becomes optimal, can be obtained. When the setting of the FFT window position is in a non-permissible range, deterioration of reception performance is large, and when the setting of the FFT window position is in a permissible range, deterioration of reception performance is small.
FIG. 2 illustrates an example of a delay profile in a case in which a delay wave 301 causes deterioration of reception performance of a main wave 300.
The main wave 300 includes a guard interval 302 and a valid symbol period 304. The delay wave 301 includes an interference component 306, a guard interval 203, and a valid symbol period 305.
In the reception environment at this time, there are 23 delay waves 301 of mode 3, guard ⅛ (guard interval 303 (=126 μs)), and having a strength of D/U (Desire to Undesire ratio)=30 dB for a signal amplitude peak of the main wave 300. In the example illustrated in FIG. 2, the delay times of the 23 waves appear every 5 μs at regular intervals from 150 μs. The environment is based on an actual example observed.
A search range of the FFT window search is within the guard interval of the main wave. An optimal window position needs to be detected based on the search result of the range. A window position, at which an amount of interference component included in the window (an amount of symbol component included in adjacent window) is minimum, is the optimal FFT window position. When the window position is not optimal, the reception performance deteriorates.
In both cases of FIGS. 1 and 2, the OFDM signal receiving system searches the received OFDM signal, and measures S/N of the received signal for every search to find the optimal FFT window position. The OFDM signal receiving system sets the window position at which the measured S/N is maximum as the optimal window position.